A vehicle may be equipped with one or more front wheels and one or more rear wheels. The vehicle may be equipped with an engine, producing shaft power to propel the vehicle. The vehicle may be equipped with a transmission for transforming shaft power from the output of an engine at relatively low torque and high speed into relatively high torque and low speed to drive one or more wheels. The vehicle may be equipped with axles for conveying shaft power from the transmission to one or more wheels. It may be advantageous, especially regarding mechanical simplicity, to drive only the front wheels or only the rear wheels using the engine, transmission, and axles. It may be advantageous, especially regarding operation in a variety of environmental conditions, to drive all of the wheels.
The force to propel a wheeled vehicle traveling at a steady speed across a level surface with no substantial wind may be represented mathematically using three terms related to the speed of the vehicle, commonly referred to as F0, F1 and F2. The force may be approximately the sum of the F0 term, the F1 term multiplied by the speed of the vehicle, and the F2 term multiplied by the square of the speed of the vehicle. The F0 term is related to dry friction, the F1 term is related to viscous friction, and the F2 term is related to aerodynamic drag. These terms are theoretically all greater than zero, and when found empirically are generally calculated, based on measurements, to be greater than zero. Thus, the force to propel the vehicle at a steady speed on a level surface with no wind is approximated by a parabolic function of that speed. The rotational torque required to drive the vehicle by traction of one or more of its wheels is therefore approximated by a parabolic function of the rotational speed of the wheels of the vehicle.
An engine producing shaft power may be capable of output across a range of rotational output speeds and across a range of output torque while burning fuel at a rate which is a predictable function of speed and torque. For example, a contemporary internal combustion reciprocating-piston or rotary-piston engine may be capable of running with acceptable smoothness and producing some amount of shaft torque output above an idle speed and may be capable of running without damage and producing some amount of shaft torque output up to a maximum engine speed. The amount of shaft torque output from the example engine may vary from a maximum value with its throttle wide open to zero with its throttle shut at a particular speed. The maximum shaft torque, commonly referred to as “the torque curve”, may be similar in magnitude, that is relatively “flat”, across a speed range that is a part of the overall speed range from idle speed to maximum engine speed.
The amount of shaft work produced for a given amount of fuel consumed, that is the efficiency of the engine in converting the potential of the fuel into shaft work, varies with operating torque and speed. The efficiency for an engine using spark-ignition and following the four-stroke cycles attributed to Otto or Atkinson is generally greatest with the throttle wide open, that is at maximum torque, and decreases to zero efficiency at zero output torque, provided that the ratio of fuel and air remains substantially the same, e.g. balanced, for all operating conditions. Enrichment of the mixture with extra fuel generally allows operating with output torque beyond the maximum that can be obtained with a balanced or lean mixture, but the efficiency of the engine is lowered by the use of this extra fuel. For a vehicle where fuel efficiency and clean exhaust are paramount, the engine generally will be controlled to operate with a substantially balanced or slightly lean mixture, for all torque levels and for all speeds except the combination of high torque and high speed which allows the engine to produce its maximum power and the speeds and torques approaching this combination.
A spark-ignition engine may be operated with alternative means of controlling or changing the torque instead of a throttle, which likewise decreases engine efficiency, though in lesser magnitude, when torque is reduced below its maximum. For instance, the engine may have cylinders equipped with intake valves, and the duration or timing of the opening or closing of these valves, or the distance of the opening of these valves, commonly referred to as “lift”, may be varied to control or to restrict the amount of air or a mixture of air and fuel, admitted to each cylinder. Changing the timing of the intake valves, so that they remain open and allow some air or air-fuel mixture to escape from each cylinder after the intake stroke, that is late intake valve closing, may result in less loss of efficiency, because the piston is not required to pull the air or air-fuel through a restriction during the intake stroke. In general, however, reducing the amount of air admitted to a cylinder below a particular level will reduce the net expansion of the gases and therefore significantly reduce the efficiency of the engine.
A compression-ignition engine is generally controlled simply by varying the amount of fuel introduced into its cylinders or other working chambers. The compression ratio is high enough and the fuel properties are such that combustion of fuel takes place around individual fuel droplets when they are introduced into the cylinder following most of the compression stroke. Maintaining a favorable mixture of fuel and air throughout the chamber to propagate a flame across the chamber from a spark source of combustion is not necessary. Therefore, air need not be restricted from entering the engine by a throttle or other means, expansion ratio is maintained, and efficiency is relatively flat across a wide range of torque values at any given speed. To change or to control the torque output of the compression-ignition engine, the amount of fuel introduced into the cylinders may be varied between zero and a predetermined maximum amount of fuel that can be burned without visible or otherwise excessive smoke or other unburned fuel in the exhaust.
A transmission is generally provided in a wheeled vehicle as part of the operative connection from the engine to the wheels. Contemporary vehicles often have a transmission, sometimes referred to as a “transaxle”, which includes one or more devices for selecting from multiple speed and torque ratios from the engine to the wheels, final drive gearing with a fixed ratio of speed and torque, and an axle differential which is connected to left and right wheels by the two halves of an axle. Such a transmission is included in a vehicle to transform the speed and torque output from the output shaft of the engine to a lesser speed and greater torque that is more suitable for turning the axle and wheels and thereby driving the vehicle. The transmission typically provides between four and eight different selectable ratios, each of which is a ratio of both the speed of the engine over the speed of the input to the final drive and the torque of the input to the final drive over the torque of the engine, not considering the drag or inertia of the transmission components. The different selectable ratios of both speed and torque are included to allow the vehicle to accelerate across a wide range of speeds and to cruise at any speed within that range above some minimum cruising speed which is customarily one third or less of the maximum cruising speed.